1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device and more particularly to a nitride semiconductor light emitting device with improved reliability at the time of high power drive.
2. Description of the Background Art
Among semiconductor light emitting devices, semiconductor laser devices are used as light sources for reading and writing of a signal of an optical recording medium such as CD (Compact Disk), DVD (Digital Versatile Disk) or Blue-Ray Disk. When a semiconductor laser device is used as a light source for writing, a higher-power semiconductor laser device is required because of increased speed and increased capacity of multi-layering media. Therefore, nitride semiconductor laser devices adapted to high power, for each wavelength of infrared, red, blue or the like, have been developed and are still now under development.
Recently, research and development has been conducted in an attempt to use a semiconductor laser device as an excitation light source for phosphors, other than a light source for reading and writing of a signal of an optical recording medium, and to use a semiconductor laser device as illumination. Even in the case where a semiconductor laser device is used as illumination, a higher-power semiconductor laser device is important to achieve higher efficiency and higher power of a semiconductor laser device.
Poor reliability resulting from degradation of a light emitting portion on a facet at the light emitting side is known as a big problem in achieving higher power of a semiconductor laser device. This is commonly known as COD (Catastrophic Optical Damage) which is a phenomenon in which the light emitting portion is thermally melted thereby causing emission stop. The optical power at which COD occurs is referred to as a COD level. The reason why COD occurs is that the light emitting portion becomes an absorption region in which laser light is absorbed. It is said that non-radiative recombination level is attributable to the absorption region.
In order to improve the COD level, generally, a window structure is formed by widening a bandgap of a light emitting portion for transmitting laser light, or a facet at the light emitting side is coated with a dielectric film for protection (see, for example, Japanese Patent Laying-Open Nos. 2002-237648 and 2002-335053).